Rapidly progressive dementia is distinguished from other neurocognitive disorders by its rapid onset (less than two years from first symptoms to diagnosis). It comprises a wide range of etiologies, including Creutzfledt-Jakob disease and Alzheimer's disease. When faced with a recent and rapid onset of major cognitive impairment, it is essential to rule out delirium or any other cause that may lead to rapid cognitive decline (such as stroke, encephalitis, or epileptic seizure). The work-up is based on a detailed history (drugs, exposure, medical history), a thorough clinical examination, and a broad biological work-up including MRI, lumbar puncture and EEG. Management should rapidly involve a neurologist or a specialized center.